ABB 615 Series ANSI, Technical Manual

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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 379
Technical Manual
Figure 183: Current flow in all the CTs for an internal fault
87LOZREF does not respond to phase-to-phase faults either, as in this case the fault
current flows between the two line CTs and so the neutral CT does not experience this fault
current.
87LOZREF is normally applied when the transformer is solidly grounded because in this
case the fault current is high enough and the ground fault can be detected easily.
Blocking based on the second harmonic of the neutral current
The transformer magnetizing inrush currents occur when the transformer is energized after
a period of de-energization. The inrush current can be many times the rated current, and
the halving time can be up to several seconds. For the differential IED, the inrush current
represents the differential current, which causes the IED to trip almost always when the
transformer is connected to the network. Typically, the inrush current contains a large
amount of second harmonics.
The blocking also prevents unwanted operation at the recovery and sympathetic
magnetizing inrushes. At the recovery inrush, the magnetizing current of the transformer
to be protected increases momentarily when the voltage returns to normal after the
clearance of a fault outside the protected area. The sympathetic inrush is caused by the
energization of a transformer running in parallel with the protected transformer already
connected to the network.
Blocking the starting of the restricted ground-fault protection at the magnetizing inrush is
based on the ratio of the second harmonic and the fundamental frequency amplitudes of
the neutral current IG_2H / IG. Typically, the second harmonic content of the neutral
current at the magnetizing inrush is higher than that of the phase currents.